1. Field of the Invention
The invention finds application in areas of printed circuit manufacture and more particularly to component lead trimming operations.
2. Description of Prior Art
In general, printed circuit assembly operations comprise the steps of component selection, component lead forming and trimming, mounting components on printed circuit boards (p.c. boards), soldering the components to the p.c. boards, trimming the soldered lead to a finished height, and testing operation of the finished assembly.
Two trimming operations may be noted, both of which involve cutting the leads to a desired length. The first takes place at the time the component leads are formed for ready insertion in the p.c. board, and may be done mechanically on a machine designed specifically for this function. The purpose of this initial cutting step is to prevent the bottom of the assembled board from becoming a tangle of long leads which would, e.g., prevent the board from being placed in the soldering bath conveyer, and which would collect an excessive amount of solder even if it could. The second trimming operation occurs after the components are soldered in place, and typically must be done by hand, which is costly.
Attempts have been made to reduce these two trimming operations to one by cutting the component leads to their final finished dimension at the time the leads are formed, thus eliminating the costly, manual cutting operation. However, when this is done the unsoldered components tend to become easily displaced from the board for lack of support, and the resulting unsoldered assembly therefore becomes unwieldy and difficult to handle.
For example, typical allowable final lead specifications call for a protrusion of the lead of approximately 50 to 60 mils from the bottom of the board. If component leads are cut to such tolerances, the component becomes top-heavy and, consequently, mechanically unstable in the board before soldering. A slight jar of the unsoldered board is sufficient to knock the component out of place. Also, with the leads so short, it is impossible to spaly them sufficiently once inserted so as to fasten them to the board. Further, even if the board is carefully placed in the soldering bath conveyor intact, the short leads tend to "float" on the molten solder. That is, the surface tension of the solder tends to push the component leads up and back through their respective lead holes. Therefore, if the leads are pre-cut to the final length in this manner, extra precautions must be taken to maintain the loose components in place until the soldering operation is complete. Such extra care is costly and time consuming.
To avoid the second trimming operation some manufacturers leave the leads long enough to avoid the above-noted problems, and resort to a clinching operation whereby the overly long component leads are folded back against the p.c. board. This provides mechanical support, but is a costly alternative and lacks a craftsman-like appearance.
As suggested above, final cutting operation is typically a manual operation, wherein an operator examines the lead lengths relative to a Manufacturing Specification and clips the leads accordingly using a hand-held plier type cutting tool known in the industry as "side cutters". Quality Control visually inspects the finished product and rejects the board with too long lead lengths. Rejection for too short a lead length also often results from these manual operations since operators often respond to the syndrome which leads a man to produce a stool from what was originally a chair which had one leg slightly shorter than the others. The printed circuit foil is often damaged by such manual cutting procedures leading to rejections and rework of the board.
It is an object of the present invention to eliminate manual trimming of leads; to produce an automated, economic, high capacity lead trimming operation; to avoid the faults of prior art techniques; and to produce a high quality finished printed circuit assembly.